Conventional alcohol-based heat transfer fluids, such as automobile antifreezes, have the tendency to corrode the metal (e.g. most notably aluminum, but also copper, iron, brass and solder) surfaces of the heat transfer system. In automobile coolant systems, any aluminum corrosion that occurs not only damages the metal surface being attacked by corrosion, but also adversely affects other parts of the coolant system due to a phenomenon called "transport deposition". By virtue of transport deposition, soluble aluminum corrosion products formed at engine surfaces are carried to and precipitated on the cooler radiator tubes, where they form heat-insulating films or coatings that impair heat-transfer.
In view of this corrosion problem, numerous corrosion inhibitors have been developed over the years for use in antifreezes, including inorganic phosphates, silicates, phosphonates, borates, azoles, siliconates, nitrates, nitrites, and the like. The need for even more effective corrosion inhibitors in antifreezes has been fostered by the changing metallurgy of vehicular cooling systems in favor of increasing use of aluminum and by smaller, harder working automotive engines having high operating engine temperatures. In addition, there is a continuing need for antifreezes and antifreeze concentrates that are stable and clear prior to and during use, thereby facilitating ease of handling by the customer or user.
Certain organophosphate esters have been disclosed for use as lubricants and corrosion inhibitors. For example, ethylene oxide-containing organophosphates and propylene oxide-containing organophosphates and a method of preparing them are disclosed in U.S. Pat. No. 4,360,474. As an additional illustration, German patent application 2,756,747 discloses PO/EO and PO/BO-(butylene oxide) containing phosphate esters (wherein "EO" denotes ethylene oxide, "PO" denotes propylene oxide, and "BO" denotes butylene oxide) useful as lubricants and corrosion inhibitors. The compounds of this publication are made by reacting polyphosphoric acid with aliphatic diols, triols, or tetrols. The organophosphates of the German application have molecular weights between 200 and 8,000. This German application is primarily directed to low-foaming corrosion inhibited metalworking lubricating fluids and does not disclose antifreeze formulations.